1. Field of Invention
The present invention relates in a general way to a process for obtaining a layer of single-crystal germanium on a substrate of single-crystal silicon or, conversely, a layer of single-crystal silicon on a substrate of single-crystal germanium.
2. Description of the Related Art
Silicon (Si) is the basic compound of microelectronics. It is currently available on the market in the form of wafers 200 mm in diameter. The performance limits of integrated circuits are in fact therefore those associated with the intrinsic properties of silicon. Among these properties, mention may be made of the electron mobility.
Germanium (Ge), which belongs to column IV of the Periodic Table of Elements, is a semiconductor. It is potentially more beneficial than Si since (i) it has a higher electron mobility, (ii) it absorbs well in the infrared range and (iii) its lattice parameter is greater than that of Si, thereby allowing heteroepitaxial structures using the semiconductor materials of columns III-V of the Periodic Table.
Unfortunately, germanium does not have a stable oxide and there are no high-diameter germanium wafers on the market, except at prohibitive prices.
Si.sub.1-x Ge.sub.x alloys have already been grown on substrates of single-crystal Si. The alloys obtained only rarely have germanium contents exceeding 50% in the alloy.
Moreover, when SiGe alloys are grown on single-crystal Si, the growth of the SiGe alloy is initially single-crystal growth. The greater the thickness of the layer and the higher its germanium content, the more the layer becomes "strained". Above a certain thickness, the "strain" becomes too high and the layer relaxes, emitting dislocations. These dislocations have a deleterious effect on the future circuits which will be constructed on this layer and the relaxation of the layers causes certain advantages of the strained band structure (offsetting of the conduction and valence bands depending on the strain states: Si/SiGe or SiGe/Si) to be lost. Corresponding to each composition and to each production temperature there is therefore a maximum thickness or strained layer.
In some applications, the concept of "relaxed substrates" has been developed, that is to say Si.sub.1-x Ge.sub.x layers are grown on silicon so as to exceed the critical thickness for a given composition, but by adjusting the deposition parameters for the layers so that the dislocations emitted do not propagate vertically but are bent over so as to propagate in the plane of the layer in order subsequently to evaporate at the edges of the wafer. Growth therefore takes place from increasingly germanium-rich layers, it being possible for the germanium gradient to change stepwise or in a continuous fashion.
However, the layers deposited by this "relaxed substrate" process either have a relatively low (&lt;50%) degree of germanium enrichment or have an unacceptable density of emergent dislocations for applications in microelectronics.
Thus, the article entitled "Stepwise equilibrated graded Ge.sub.x Si.sub.1-x buffer with very low threading dislocation density on Si (001), by G. Kissinger, T. Morgenstern, G. Morgenstern and H. Richter, Appl.Phy.Lett. 66(16), Apr. 17, 1995", describes a process in which the sequence of the following layers is deposited on a substrate:
250 nm Ge.sub.0.05 Si.sub.0.95 +100 nm Ge.sub.0.1 Si.sub.0.9 +100 nm Ge.sub.0.15 Si.sub.0.85 +150 nm Ge.sub.0.2 Si.sub.0.8. PA1 a) in the case of deposition of the layer of single-crystal germanium, the deposition temperature is gradually reduced in the range of 800.degree. C. to 450.degree. C., preferably 650 to 500.degree. C., while at the same time gradually increasing the Ge/Si weight ratio in the precursor gas mixture from 0 to 100%; and PA1 b) in the case of deposition of the layer of single-crystal silicon, the deposition temperature is gradually increased in the range of 450 to 800.degree. C., preferably 500 to 650.degree. C., while at the same time gradually increasing the Si/Ge weight ratio in the precursor gas mixture from 0 to 100%. PA1 (i) holes which have a depth not exceeding the upper constant-composition layer. In order to obviate these holes, a sufficient thickness of the desired final composition of the upper layer (typically pure Ge) must be deposited and polished until the "holes" disappear; PA1 (ii) "deeper" holes in the form of an upside-down pyramid [on a substrate of (100) single-crystal Si]; the apex of the upside-down pyramid lies in the graded-composition layers, usually those with a Ge concentration above 55%. These holes, with sharp rectangular edges, are rounded in various ways by the polishing, but to the detriment of a greater region of extension. In order to limit these holes, one technique includes introducing, into the growth stages above a Ge concentration of 55%, steps in which the composition is constant (typically 300 nm every 10%, thereby increasing the process time per wafer, which time nevertheless remains acceptable). This has the effect of reducing the density of these defects; PA1 (iii) another possibility for limiting the extension of the "deeper" holes includes in fabricating a substrate with concentrations of up to about 70% and polishing, cleaning and resuming the growth by increasing the Ge concentration of the layers up to that desired. This may be carried out as many times as necessary. Thus, lower defect densities and a lower extension of these defects are obtained. PA1 Polishing machines and solutions conventionally used in silicon technology may be used for chemical-mechanical polishing; PA1 Only very little material need be polished, this furthermore appearing as peak-to-valley material (in general about 200 nm). This leaves a great deal of freedom and allows the use of CMP solutions identical to those for silicon; PA1 The disappearance of the "work-hardened" region after polishing is easy to achieve since compounds of Ge with oxygen are very unstable, i.e. they dissolve in any process containing oxygen (thermal oxidation or plasma-assisted oxidation, dissolution in ozonated water or in chemical solutions which selectively etched Ge, etc.) PA1 It involves "light" polishing, that is to say a process with a high degree of freedom in the choice of thickness and of uniformity, if care is taken to initially produce a relatively thick layer to be polished (typically more than twice the peak-to-peak roughness to be removed); PA1 Repeating the epitaxial growth of a layer having the same composition as that which has just been polished will "guarantee" the surface and make it possible to continue with other variations (other relaxed or strained layers above it), that is to say the starting surface is one without any roughness; PA1 This technique may be extended to the reverse situation, that is to say to the formation of Si layers on Ge.
After each layer has been deposited, it undergoes in situ annealing in hydrogen at 1095 or 1050.degree. C. By way of comparison, similar sequences of layers have been deposited, but without annealing.
A 300 nm layer of Ge.sub.x Si.sub.1-x of the same composition as the upper buffer layer is also deposited on the latter.
The specimens which did not undergo intermediate annealing have an emergent-dislocation density of 10.sup.6 cm.sup.-2, whereas the specimen which underwent annealing has an emergent-dislocation density of 10.sup.3 -10.sup.4 cm.sup.-2.
The article entitled "Line, point and surface defect morphology of graded, relaxed GeSi alloys on Si substrates", by E. A. Fitzgerald and S. B. Samavedam, Thin Solid Films, 294, 1997, 3-10, describes the manufacture of relaxed substrates comprising up to 100% germanium. However, the process employed takes a long time (more than about 4 hours per wafer) and is consequently unattractive from an industrial standpoint. Moreover, this process is not reversible, that is to say it does not allow pure silicon to be deposited on a germanium substrate.
Furthermore, during the fabrication of such relaxed substrates, a surface roughness is observed which increases depending on the deposition conditions and which may have negative defects--since they are cumulative--that is to say an onset of roughness can but increase during definition.